A DC arc furnace usually operates with a cathodic graphite or Soderberg type arcing electrode with the charge in the furnace hearth functioning as the anode and heated by the arc. It is necessary to transmit the electric power through the charge, arc, and arcing electrode, requiring an electrical power connection with the charge.
Normally the furnace is charged initially with solid metal pieces, exemplified by steel scrap, and during start-up a movable starting electrode connected to the power circuit is pressed against the charge to provide the electrical connection. After melting to a degree forming a pool of melt of adequate depth, the electrical connection is made via a melt electrode in the form of a metal bar encased by refractory with its inner end exposed to the melt and its outer end externally exposed and connected to the power circuit.
When the arcing electrode is cathodic, it is connected with a negative power line while the starting and melt electrodes are connected together to a positive line. For power both lines are connected to the appropriate poles of the DC power supply which is normally an inverter for AC current.
As the forming melt contacts the inner end of the melt electrode, the starting electrode is withdrawn so that it is not damaged, it being normally made of metal at least mainly and not being intended for use other than the start-up. The melt electrode is also made of metal but it is designed so that although its inner end and a portion extending from that end become molten, its outer end and a portion extending inwardly from that end are maintained in an unmelted state by adequate cooling of the refractory encasing the bar and cooling of the bar's outer end. The bar can be made of a metal compatible with that of the charge in the furnace; in the case of a steel charge the bar can be made of steel of generally corresponding analysis.
Some of the details of the start-up and then production operation of a DC arc furnace are described by the Valchev et al. U.S. Pat. No. 3,835,230, Sept. 10, 1974, although the details of a workable melt electrode are not disclosed there. Such details are disclosed by the Stenkvist U.S. Pat. No. 3,997,712, Dec. 14, 1976, and by the Andersson Application Ser. No. 744,423, filed Nov. 23, 1976, as a continuation-in-part of application Ser. No. 634,061 filed Nov. 21, 1975 (since abandoned), the Stenkvist patent and Andersson application both being assigned to the assignee of the present invention.
In the Valchev et al. patent switches are used to switch the power off from the starting electrode and on the melt electrode at the appropriate time; it is also possible to keep both electrodes connected together to, for example, the positive power line, because after the melt electrode is in good contact with the melt the starting electrode need only be removed, the current then commutating automatically to the melt electrode.
The present inventor has concerned himself with the problem that if the starting electrode does not make a good electrical contact with the initially charged metal pieces or scrap, an arc is formed between the starting electrode and the charge as the starting electrode transmits the power to the charge. This causes undesirable rapid wear of the starting electrode, and the consumption of power where such consumption is not desired.
Of even more importance, if the starting electrode is withdrawn before the formation of a melt of sufficient depth to make a good contact with the melt electrode, overheating and possibly the formation of an arc between the melt and the melt electrode can occur, such as between the inner end of the melt electrode and the inadequately formed melt. This may also occur even if the melt depth is adequate for a good electrical contact with the melt electrode, if the furnace is tilted too early for slag removal, an electric arc furnace normally being a tilting furnace. Such possible arcing between an inadequate melt and the starting electrode can cause heavy wear on the refractory encasing the melt electrode, possibly requiring a shutdown of the furnace for repair of this refractory. Once the starting electrode is removed from the charge, the melt electrode must carry all the power, emphasizing the necessity for a good electrical contct with the melt and the risk of rapidly wearing or possibly destroying the melt electrode's refractory if arcing occurs because of a poor contact.